Modified cycloidal impeller



United States Patent G 3,371,856 MODIFIED CYCLOIDAL IMPELLER Francis H. Thelen, Tujunga, and Thomas M. Thomas,

Downey, Calif, assignors to Fuller Company, Catasauqua, Pa, a corporation of Delaware Filed Mar. 24, 1966, Ser. No. 537,209 2 Claims. (Cl. 23lll41) This invention relates to interrneshing impellers for rotary positive displacement blowers and the like and in particular to the contour of the profile of such impellers.

Blowers and the like of the Roots type employ a pair of intermeshing lobed impellers which rotate in opposite directions within a casing. The impellers are identical, and each is constructed with a waist portion defined by concave surfaces and with radially extending lo'be portions defined by convex surfaces. In operation the impellers are mounted on parallel shafts which are driven in synchronization so that a lobe of one impeller always moves through the waist portion of the mating impeller. Conventionally, each shaft is provided with a timing gear which meshes with the other timing gear so as to maintain synchronization of the shafts and impellers. It is, of course, necessary that each impeller rotate through the same an gular amount per unit of time, and the timing gears are therefore identical in size. It is also true in this type of arrangement that the angle between the two lines drawn from the center of the impeller (that is, the axis of the shaft) through the points of intersection of the convex surface of a lobe portion with the concave surfaces of the waist portion is equal to 360/2n, where n is the number of lobes. Thus, the total convex surface of each lobe of a two-lobe impeller subtends an angle at the axis of the impeller shaft equal to 90, and for a three-lobe impeller, 60.

The design of the profiles of the waist and lobe portions of this general kind of impeller depends somewhat on the use to which the equipment is to be subjected, because the characteristics required for one use may be inconsistent with the characteristics required for another use. For example, maximum displacement has been achieved in the past with impellers having lobes whose convex profiles are involute curves, and such impellers operate very efiiciently as blowers for gases which are free of liquid particles. This type of impeller, however, forms small pockets between lobe and waist which trap and compress portions of the fluid medium being handled. This characteristic renders the impellers unsuitable for handling gases Which contain particles of liquid, because the incompressibility of the liquid will result in mechanical damage to the impellers. Also, the entrapment of fluid results in poor efficiency when the impellers are employed to produce high vacuum. To avoid the disadvantage of fluid entrapment by impellers it is known to construct impellers with loibe profiles which are cycloidal curves. These impellers avoid fluid entrapment because they make only point contact with each other. However, this eflect is accompanied "by a lowered displacement.

Impellers which represent a comprise between the abovementioned involute and cycloidal impellers are suggested in Patent No. 3,089,638 issued May 14, 1963. The impellers described in the patent are designed to have substantially only point contact with each other and to exhibit only a small reduction in displacement from that which would result from true involute design. Specifically, each impeller is constructed with 21 waist portion defined by concave circular arcs and with lobe portions which are 3,371,856 Patented Mar. 5,1968

non-uniformly convex, the lobe portions being generated from the locus of the waist arcs as the impellers are rotated. The impellers are constructed by first calculating the necessary waist radius by means of a formula which relates the waist radius to the number of lobes, the thickness of the waist and the gear pitch diameter of the timing gears employed to rotate the impellers in synchronization.

It is the primary object of the present .invention to provide a lobed impeller which in use with a mating impeller gives both point contact and high displacement, the impeller differing from conventional cycloidal and involute impellers and from the previously suggested compromise impeller in having a circular lobe profile and a non-circular waist profile and in having the circular lobe developed in accordance with a formula which results in an improved overall design.

The invention will be further understood from the following detailed description taken with the drawing in which:

FIGURE 1 is a diagrammatic sectional view through a blower which embodies the principles of the present invention;

FIGURE 2 is a diagrammatic view of an impeller blank before the final contours are formed; and

FIGURE 3 is a diagrammatic view of the impeller blank of FIGURE 2 during cutting of a waist contour.

Referring to FIGURE 1 there is shown in diagrammatic form a blower 10 which includes a pair of identical two-lobed impellers 12, 14 and a casing 16 having an inlet 18 and an outlet 20. The impellers are mounted at their centers on respective shafts 22, 24 which carry identical intermeshing timing gears 26, 28.

In the position illustrated, the impellers have been rotated in the direction of the arrows through an arc of 45 from the position in which the lower tip of the impeller 14 was in contact with the central point of the waist of the impeller 12. In this position the impellers are in contact with each other at a point which on each impeller lies at the intersection of the concave waist surface with the adjoining convex lobe surface. This relationship is typical in the kind of impeller under consideration and results from the inherent characteristics of the arrangement.

According to the principles of the present invention the convex surfaces of the lobe portions of the impellers are constructed as equal circular arcs, and the concave surfaces of the waist portions are cnostructed as noncircular arcs whose contours mate with the lobe of the other impeller to give point contact between the two during use. At the same time it is necessary, of course, that the junctions of the concave and convex surf-aces be located such that the pairs of impellers continuously roll against one another as is typical in this general kind of impeller. It will be seen that it is not practical merely to select a lobe radius at random and then construct the waist contours, because this procedure will not necessarily result in the proper location of the junctions between concave and convex surfaces. According to the present invention the tip diameter of the impeller, that is the distance from the tip of one lobe to the tip of the other lobe along the center line of the impeller, is selected first, the criterion for this selection being the desired overall size of the final blower or the inside dimension of a given housing in which the impeller is to rotate. Following the selection of this dimension the lobe radius, as measured radially inwardly from the lobe tip along the center line of the impeller, is calculated by a formula which relates the lobe radius to the tip diameter and to the shaft center distance (equivalent to the timing gear pitch diameter). The necessary waist contours may be marked out by first forming the lobes on a pair of impeller blanks and then rotating the blanks together so that the locus of the tip of a lobe of one impeller defines the waist contour on the other impeller. Alternatively, the necessary waist contour can be converted to X and Y coordinates and these figures subsequently are employed to locate a cutter in a sequence of positions.

Referring now to the formula by which the lobe radius is calculated it will be seen that in the position of the impellers in FIGURE 1 the point of contact G between the impellers 12 and 14 lies midway between the axes of the shafts 22, 24. Thus, if the shaft center distance is referred to as CD, the distance from point G to each shaft axis is CD/ 2. The tip diameter is illustrated by TD and the tip radius by TR. Since TR will vary with TD, the distance between the shaft axis E and the center of curvature F of each lobe will vary. In terms of TD and TR this distance g is equal to If the points E, F and G are connected by straight lines e, f and g to form a triangle, the angle at E may be expressed trigonometrically as In the illustrated position of the impellers E:45, e=TR, f=CD/ 2 and cos [1:

because of relationships previously noted for this general type of impeller. If these values are substituted into the above equation er ee rcos 45 (Z2 TP) Simplifying and solving for TR:

for cos 45 where n is the number of lobes.

FIGURES 2 and 3 illustrate a preferred manner of forming the waist contours of an impeller after the lobe tip radius TR has been calculated. FIGURE 2 shows an impeller blank 30 which has been cut in a size somewhat larger than the final impeller, the contours of the latter being shown in dotted lines. In the particular embodiment illustrated both the lobes and the waist have been cut with circular surfaces. The location of the points F together with the magnitude of the final lobe radius TR will have been previously determined, and the blank 30 has been cut, for example, with a lobe radius about 0.1 inch greater than the final radius. The waist curvatures of the blank 30 are preferably cut so that the deepest point on the circular arc isdirectly opposite the center of the impeller. The circular arc is, of course, more concave thanthe curvature to be cut subsequently.

Two impeller blanks are then stacked one on top of the other on a jig borer rotary table and are subsequently cut with a rotary cutter along their waist portions by making a series of cuts each at a different angular position of the impeller blanks and at a dififerent horizontal position of the cutter. As shown in FIGURE 2, an initial cut is being made on the horizontally disposed blanks with a cutter 32 which is rotatable about a vertical axis The next cut is made after rotating the blanks counterclockwise through a small angle, for example A2", and after horizontally positioning the cutter 32 according to a precalculated tabulation. The position of the cutter 32 will depend, of course, on the are through which the blanks are rotated for that cut. Conveniently, the cutter p0Sition is arrived at by employing an X and Y grid over which the axis of the cutter may be adjusted relative to a reference line and by employing a table which gives the necessary x and y coordinates for each arc of rotation of the impeller blanks. As shown for an angle A the x and y values are positive thus locating the axis of the cutter 32 at Z. The table is arrived at ahead of time by calculation of the center of the locus of a lobe tip of one impeller relative to the waist of an opposed im-' peller during simultaneous opposite rotation of the two. A plurality of cuts are made in sequence until the impeller blanks have been rotated 45. The procedure is then repeated as the blanks are rotated stepwise and clockwise from the initial position, and the x and y coordinates are given negative values. The opposite waist profile is cut by turning the blanks and repeating the sequential cuts with positive coordinates and then with negative coordinates.

Following cutting of the waist curvatures the lobe curvatures are formed by grinding to the calculated circular configuration. It is possible, alternatively, to grind the lobe curvatures first and to mark out the waist curvatures by using the lobes similar to templates. In this procedure the blanks would be mounted on parallel shafts in offset relationship and the shafts rotated in opposite directions in synchronization in increments. Simultaneously, the profile of the lobe tip of one blank would be traced on the waist portion of the opposite blank to mark out the proper concave profile of the latter. The waist would then be cut by conventional techniques.

The drawing is illustrative of the principles of the in vention and the described details are not intended to be limiting except as they appear in the appended claims.

What is claimed is:

1. Fluid handling apparatus of the rotary positive dis placement type comprising a pair of lobed impellers mounted on parallel shafts and rotatable at a constant velocity ratio, each of said impellers having a central waist portion adapted to be mounted on one of said shafts and a plurality of identically shaped lobe portions extending radially outwardly from said waist portion, the profile of each of said impellers consisting of a like plurality of identical convex circular arcs of uniform radius defining said lobe portions and a like plurality of identical concave non-circular curves defining said waist portion, each of said arcs subtending an angle at the axis of the impeller shaft equal to 360/2n, where n is the number of said lobe portions, and each of said non-circular curves being generated from a circular arc identical with those defining said lobe portions so that, when said impellers are rotated in opposite directions at the same velocity, there is substantially point contact between said impellers at all positions of relative rotation.

2. For use in fluid handling apparatus of the rotary positive displacement type having a pair of lobed impellers mounted on parallel shafts and rotatable at a constant velocity ratio, an impeller having a central waist portion coaxial with the associated shaft and a plurality of identically shaped lobe portions not exceeding three in number extending radially outwardly from said waist portion, the profile of said impeller consisting of a like plurality of convex circular arcs of uniform radius defining said lobe portions and a like plurality of concave non-circular curves defining said waist portion, each of said circular arcs subtending an angle at the axis of said associated shaft equal to 360/2n, where n is the number of said lobe portions, each of said non-circular curves being generated from a circular are identical with those defining said lobe portions, the radius TR of each of said circular arcs being so related to the shaft center distance CD, the impeller tip diameter TD and the number n of said lobe portions that References Cited UNITED STATES PATENTS Wendell 103-426 Flanagan 230-141 Brun 103-126 Rose 230141 Hubrich 103-126 Lorenz 230-141 10 DONLEY J. STOCKING, Primary Examiner.

WILBUR J. GOODLIN, Examiner. 

1. FLUID HANDLING APPARATUS FOR THE ROTARY POSITIVE DISPLACEMENT TYPE COMPRISING A PAIR OF LOBED IMPELLERS MOUNTED ON PARALLEL SHAFTS AND ROTATABLE AT A CONTACT VELOCITY RATIO, EACH OF SAID IMPELLERS HAVING A CENTRAL WAIST PORTION ADAPTED TO BE MOUNTED ON ONE OF SAID SHAFTS AND A PLURALITY OUTWARDLY FROM SAID WAIST PORTION, THE EXTENDING RADIALLY OUTWARDLY FROM SAID WAIST PORTION, THE PROFILE OF EACH OF SAID IMPELLERS CONSISTING OF A LIKE PLURALITY OF IDENTICAL CONVEX CIRCULAR ARCS OF UNIFORM RADIUS DEFINING SAID LOBE PORTIONS AND A LIKE PLURALITY OF IDENTICAL CONCAVE NON-CIRCULAR CURVES DEFINING SAID WAIST PORTION, EACH OF SAID ARCS SUBTENDING AN ANGLE AT THE AXIS OF THE IMPELLER SHAFT EQUAL TO 360*/2N, WHERE N IS THE NUMBER OF SAID LOBE PORTIONS, AND EACH OF SAID THAT, WHEN CULAR CURVES BEING GENERATED FROM A CIRCULAR ARC IDENTICAL WITH THOSE DEFINING SAID LOBE PORTIONS SO THAT, WHEN SAID IMPELLERS ARE ROTATED IN OPPOSITE DIRECTIONS AT THE SAME VELOCITY, THERE IS SUBSTANTIALLY POINT CONTACT BETWEEN SAID IMPELLERS AT ALL POSITIONS OF RELATIVE ROTATION. 